This was pretty fun lol. I would suggest a few things though.
1. The turning speed is horribly slow, it took me way to long to do a full turn around.

2. Shooting the zombies was kinda difficult. The cross hairs didn't line up with their heads until they were almost right in front of me ( and they couldn't be killed until it was lined up). So, when They got close enough, I was backing up to avoid getting hit. The zombie was following me at the exact same speed that I was moving.
Maybe you could make the zombies slightly larger, so the cross hairs can be on their heads without being directly in front of you. And also maybe make the zombies move at a slightly slower speed. Right now, You could easily cheat by constantly moving backwards, and the zombies could never be further or closer to you. I kept finding myself moving towards the zombie to get close enough to shoot them, and the backing up constantly until i could get the killing shot.

I'm not sure if that made much sense, but I'm sure others will understand what I mean when they play it.

Other than that I think it's awesome. It will also be much better if you add a walking animation for the zombies!

Not bad! It's surprising that we haven't had one of these yets. It was actually quite fun. I agree with above about that turning speed. If you increase that and make the hit markers a bit more effective, this could be much more exciting. Also, add a few more zombies instead of just 2 because that gets a little boring after a while. Also, try testing the cash amounts when every kill is $10 instead of $5. That would make the excitement last longer with each new weapon. Keep up the good work!